1. Field of the Invention
This invention relates generally to refrigeration compressors, and more particularly to an unloading mechanism for refrigeration compressors, especially multi-cylinder compressors.
2. Description of the Prior Art
It has been common practice to provide means for unloading a multi-cylinder refrigeration compressor in order to avoid starting the compressor under load, and further to control the refrigeration capacity in response to the cooling demand. Prior compressor unloading mechanisms known to the present applicants have generally fallen into two different categories. In the first category, means are provided for holding the suction valves open so that the refrigerant is drawn into a cylinder and then immediately discharged therefrom through the suction valve rather than being compressed and discharged through the discharge valve; U.S. Pat. No. 3,144,982 shows an unloading mechanism typical of the first category. In the second category, means is provided for blocking the suction passage so that no refrigerant is drawn into the cylinder; U.S. Pat. No. 3,578,883 shows an unloading mechanism typical of the second category. Prior unloading mechanisms in both categories known to the present applicants have been characterized by appreciable complexity and thus expense and it is therefore desirable to provide a simple unloading mechanism in the second or blocking category which is readily controlled by a signal external to the compressor.